Re: Crossing loop design (r.e. Southern Aurora crash)

["Notagunzel" <notagunzel@bigfoot.com>]

As far as I'm concerned all of this boils down to SPADS & overlaps. Overseas
(British?) experience is that in most SPADs (Signals Passed at Danger) the
train pulls up within a fairly short distance.  In a crossing loop,
therefore, running crosses are relatively safe if there are overlaps beyond
the Departure Home.  On the Western CTC, the approach release is to 'force'
the Driver down to 40km/h, so if the driver misjudges the braking, the train
should still pull up within the short (100m?) overlap.  On the newer
suburban loops, Laverton Loop & Lyndbrook etc... the overlap is long, long
enough to pull up a spark running at line speed when it trips the Departure
signal. The problem with the NE CTC is that there is no overlap, with
running crosses under low speed aspects.  This is historical, witness Rock
and Drome loops (long deceased).  The lack of overlap is exaccerbated if
drivers enter NE CTC loops at more than 15 km/h. (Is it true it's usually
closer to 40km/h?) NSW doesn't have running crosses, with catch points in
the loop (very British Board of Trade), so crosses are slower than a running
cross.  SA and WA don't bother with overlaps at loops, WA doesn't have
running crosses, only a time release on the opposing home (like Manor etc..)
AFAIK SA does have running crosses.  Of course with the looser designs you
have to put up with the odd Hines Hill now and again.  In the reported
details of the Coroners Report into Hines Hill, the Coroner appered to show
little understanding of Signal Design.  (Recomending 2 Red signals behind a
train shows a misunderstanding of overlaps in places with irregularrly
spaced signals)

British Overlap is 183 metres, Victorian Overlap in Suburban area is
emergency braking distance when running at the speed of the previous aspect,
and in country areas varies a bit.  On the NE CTC the overlap for autos can
be just a Floater Track Relay, with no real defined cut off.  (A FTR is a
relay across the feed, which picks up when the train shunt receedes into the
track circuit).  The US usually doesn't bother with overlaps.  (Just have
spectacular prangs)


Victoria's phobia of slips... we call them catches or catchpoints in
Victoria, a slip is a fielder in Cricket, or something R classes do going up
hills?  :o)  Anyway, as has been said, there are few/nil catches for
passenger running, they are used for 'roll out protection' from sidings
only.  The only catches in running lines I know of are on the thru goods
lines at Viaduct Jct & Sunshine.

Homes in the Underground?  Gee the drivers can't see the next train outside
(Newport) :o)

BC